Copper foil is used in the production of printed circuit boards. Although an excellent electronic conductor, there are problems inherent with the use of such foil. Copper is easily oxidized and corroded. In the production of printed circuit boards, it is generally necessary to bond the copper foil to dielectric substrates to provide the foil with dimensional and structural stability. As plated or rolled, the adhesion of copper foil to such substrates is generally insufficient. Copper is also known to accelerate or catalyze the decomposition of dielectric substrates. For these reasons, copper foil is typically sold with one or more protective layers applied to its surface.
The current practice for applying protective layers to copper foil typically involves the following sequence of steps. First: a nodularized or dendritic copper layer is deposited on the foil surface. This dendritic layer can be applied to either the matte side or the shiny side of the foil, or to both sides of the foil. The dendritic layer is applied to increase mechanical interlocking between the dielectric substrate and foil surface to thereby increase the adhesion strength of the foil. Second: a barrier layer typically comprised of brass is then deposited on the dendritic layer of copper. This barrier layer is added to prevent thermal degradation of the metal-resin interface, thereby maintaining adhesion of the foil to the resin. Third: a stabilization layer typically comprised of zinc and chrome is then applied to both sides of the foil. The stabilization layer aids in oxidation resistance, shelf-life and humidity durability.
The foregoing practice has a number of disadvantages. The nodularized copper layer increases foil profile as well as the etching time required to etch circuitry using the foil. The nodularized layer also decreases foil quality by increasing the pit and dent count, and it reduces treat line speed. The application of the barrier layer requires the use of a caustic, cyanide-containing bath which is difficult and costly to waste treat. Application of the barrier layer also requires the use of soluble anodes which contribute to poor foil quality and anodes that are susceptible to polarization. During the application of the stabilization layer undesirable precipitates form in the bath. The present invention overcomes many of these problems by providing a copper foil that does not require the nodularized or dendritic copper layers or the barrier layers required by prior art copper foils, but still possess initial peel strengths and thermal degradation resistance properties that are comparable to prior art foils.
The dielectric substrates used in the market place, which are sometimes referred to as prepregs, are often made using epoxy resins. Many epoxy resin based prepregs that are available are made using amine curing agents, such as dicyandiamine. There are however, a number of problems associated with the use of such amine curing agents including environmental, safety and handling concerns. Recently, new prepregs based on epoxy resin systems that are made without such amine curing agents have been introduced into the market place. These new epoxy prepregs are sometimes referred to as "non-dicy" prepregs. While these non-dicy prepregs are beneficial, a problem with the use of such prepregs relates to the fact that the initial peel strength between the copper foil and the non-dicy prepreg that is typically achieved is generally less, in some instances about 10% less, than when conventional epoxy prepregs are used. The present invention also overcomes this problem by providing a treated copper foil that can be used with non-dicy prepregs and still provide desired initial peel strength levels.